Transalkylation of heavy aromatic hydrocarbon feedstocks

ABSTRACT

In a process for producing xylene by transalkylation of a C 9 + aromatic hydrocarbon feedstock with a C 6  and/or C 7  aromatic hydrocarbon, the C 9 + aromatic hydrocarbon feedstock, at least one C 6  and/or C 7  aromatic hydrocarbon and hydrogen are contacted with a first catalyst comprising (i) a first molecular sieve having a Constraint Index in the range of about 3 to about 12 and (ii) at least first and second different metals or compounds thereof of Groups 6 to 12 of the Periodic Table of the Elements. Contacting with the first catalyst is conducted under conditions effective to dealkylate aromatic hydrocarbons in the feedstock containing C 2 + alkyl groups and to saturate C 2 + olefins formed so as to produce a first effluent. At least a portion of the first effluent is then contacted with a second catalyst comprising a second molecular sieve having a Constraint Index less than 3 under conditions effective to transalkylate C 9 + aromatic hydrocarbons with said at least one C 6 -C 7  aromatic hydrocarbon to form a second effluent comprising xylene.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/301,055 filed Feb. 3, 2010, the disclosure of which is fullyincorporated herein by reference.

FIELD

This invention relates to transalkylation of heavy (C₉+) aromatichydrocarbon feedstocks to produce xylene.

BACKGROUND

An important source of xylene in an oil refinery is catalytic reformate,which is prepared by contacting a mixture of petroleum naphtha andhydrogen with a strong hydrogenation/dehydrogenation catalyst, such asplatinum, on a moderately acidic support, such as a halogen-treatedalumina. Usually, a C₆ to C₈ fraction is separated from the reformateand extracted with a solvent selective for aromatics or aliphatics toproduce a mixture of aromatic compounds that is relatively free ofaliphatics. This mixture of aromatic compounds usually contains benzene,toluene and xylenes (BTX), along with ethylbenzene.

However, the quantity of xylene available from reforming is limited andso recently refineries have also focused on the production of xylene bytransalkylation of C₉+ aromatic hydrocarbons with benzene and/or tolueneover noble metal-containing zeolite catalysts. One such process, usingMCM-22 as the zeolite catalyst is disclosed in U.S. Pat. No. 5,030,787.However, during the transalkylation of C₉+ aromatics with, for example,toluene to produce xylene and benzene, saturated by-products, which boilin the same temperature range as the desired aromatic products, aretypically produced making separation of the desired products at highpurity levels difficult. For example, a commercial benzene product mayneed a purity of 99.85 wt % or higher. However, initial benzene purityafter distillation of a transalkylation reaction product is typicallyonly 99.2% to 99.5% due to the presence of coboilers, such asmethylcyclopentane, cyclohexane, 2,3-dimethylpentane,dimethylcyclopentane and 3-methylhexane. Therefore, an additionalextraction step is usually required to further improve benzene productpurity to the desired level.

One solution to the problem of the production of benzene co-boilersduring the transalkylation of heavy aromatics is disclosed in U.S. Pat.No. 5,942,651 and involves the steps of contacting a feed comprising C₉+aromatic hydrocarbons and toluene under transalkylation reactionconditions with a first catalyst composition comprising a zeolite havinga constraint index ranging from 0.5 to 3, such as ZSM-12, and ahydrogenation component. The effluent resulting from the firstcontacting step is then contacted with a second catalyst compositionwhich comprises a zeolite having a constraint index ranging from 3 to12, such as ZSM-5, and which may be in a separate bed or a separatereactor from the first catalyst composition to produce a transalkylationreaction product comprising benzene and xylene. A benzene product havinga purity of at least 99.85% may be obtained by distilling the benzenefrom the transalkylation reaction product, without the need for anadditional extraction step. According to the '651 patent, the secondcatalyst composition comprises up to 20 wt % of the total weight of thefirst and second catalyst compositions.

Another problem associated with heavy aromatics alkylation processes iscatalyst aging since, as the catalyst cokes with increasing time onstream, higher temperatures are normally required to maintain constantconversion. When the maximum reactor temperature is reached, thecatalyst needs to be replaced or regenerated. Depending on the C₉+:C₆ orC₇ composition of the feed, the cycle length may vary from only 9 monthsfor high C₉+:C₇ ratios of 85:15 to about 5 years for low C₉+:C₇ ratiosof 20:80. Recent work has shown that the aging rate of existingtransalkylation catalysts is also strongly dependent on the presence inthe feed of aromatic compounds having alkyl substitutents with two ormore carbon atoms, such as ethyl and propyl groups. Thus these compoundstend to undergo disproportionation to produce C₁₀+ coke precursors.

To address the problem of C₉+ feeds containing high levels of ethyl andpropyl substituents, U.S. Published Application No. 2009/0112034discloses a catalyst system adapted for transalkylation a C₉+ aromaticfeedstock with a C₆-C₇ aromatic feedstock comprising: (a) a firstcatalyst comprising a first molecular sieve having a Constraint Index inthe range of 3-12 and 0.01 to 5 wt % of at least one source of a firstmetal element of Groups 6-10; and (b) a second catalyst comprising asecond molecular sieve having a Constraint Index less than 3 and 0 to 5wt % of at least one source of a second metal element of Groups 6-10,wherein the weight ratio of said first catalyst to said second catalystis in the range of 5:95 to 75:25. The first catalyst, which is optimizedfor dealkylation of the ethyl and propyl groups in the feed, is locatedin front of said second catalyst, which is optimized fortransalkylation, when they are brought into contact with said C₉+aromatic feedstock and said C₆-C₇ aromatic feedstock in the presence ofhydrogen.

Whereas the multiple catalyst bed system of U.S. Published ApplicationNo. 2009/0112034 represents a significant improvement over singlecatalyst bed processes for the transalkylation of heavy aromatic feeds,it suffers from the disadvantage that the first catalyst, in addition todealkylating ethyl and propyl substituted aromatics and saturating theresultant olefins, tends to facilitate the side reaction of aromaticssaturation. This reaction is primarily a concern at the start of cyclewhen the reactor temperature is at its lowest and is undesirable sinceit reduces aromatic yield and generates saturated products. In addition,some of these saturated products, e.g. cyclohexane andmethylcyclohexane, have boiling points close to benzene which makes itdifficult to recover high purity benzene. Although this problem can bealleviated by sulfiding the metal catalysts on start-up to reduce theiraromatic saturation activity, this is often undesirable as itnecessitates adding the facilities and sulfiding agents to effect themetal sulfidation.

The present invention seeks to provide a C₉+ aromatic transalkylationprocess that retains the advantages of the multiple bed catalyst systemwhile reducing the problem of aromatic saturation without the need forpre-sulfidation of the catalysts.

SUMMARY

In one aspect, the invention resides in a process for producing xyleneby transalkylation of a C₉+ aromatic hydrocarbon feedstock with a C₆and/or C₇ aromatic hydrocarbon, the process comprising:

(a) contacting a C₉+ aromatic hydrocarbon feedstock, at least one C₆and/or C₇ aromatic hydrocarbon and hydrogen with a first catalyst underconditions effective to dealkylate aromatic hydrocarbons in thefeedstock containing C₂+ alkyl groups and to saturate C₂+ olefins formedso as to produce a first effluent, the first catalyst comprising (i) afirst molecular sieve having a Constraint Index in the range of about 3to about 12 and (ii) at least first and second different metals orcompounds thereof of Groups 6 to 12 of the Periodic Table of theElements; and then

(b) contacting at least a portion of said first effluent with a secondcatalyst comprising a second molecular sieve having a Constraint Indexless than 3 under conditions effective to transalkylate C₉+ aromatichydrocarbons with said at least one C₆-C₇ aromatic hydrocarbon to form asecond effluent comprising xylene.

Conveniently, first metal is at least one of platinum, palladium,iridium, and rhenium and the second metal is at least one of copper,silver, gold, ruthenium, iron, tungsten, molybdenum, cobalt, nickel, tinand zinc.

In one embodiment, the first metal comprises platinum and said secondmetal comprises copper.

Conveniently, the first metal is present in the first catalyst in amountbetween about 0.001 and about 5 wt % of the first catalyst and thesecond metal is present in the first catalyst in amount between about0.001 and about 10 wt % of the first catalyst.

Conveniently, said first molecular sieve comprises at least one ofZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-57 and ZSM-58.Conveniently, the second molecular sieve comprises at least one ofzeolite beta, zeolite Y, Ultrastable Y (USY), Dealuminized Y (Deal Y),mordenite, NU-87, ZSM-3, ZSM-4 (Mazzite), ZSM-12, ZSM-18, MCM-22,MCM-36, MCM-49, MCM-56, EMM-10, EMM-10-P and ZSM-20. In one embodiment,the said first molecular sieve is ZSM-5 and the second molecular sieveis ZSM-12.

Conveniently, the first molecular sieve has an alpha value in the rangeof 100 to 1500 and the second molecular sieve has an alpha value in therange of 20 to 500.

Conveniently, the second catalyst also comprises the same first andsecond metals or compounds thereof as the first catalyst.

Conveniently, the weight ratio of the first catalyst to the secondcatalyst is in the range of 5:95 to 75:25.

Conveniently, the conditions employed in the contacting (a) and (b)comprise a temperature in the range of about 100 to about 800° C., apressure in the range of about 790 to about 7000 kPa-a, a H₂:HC molarratio in the range of about 0.01 to about 20, and a WHSV in the range ofabout 0.01 to about 100 hr⁻¹.

Conveniently, the process further comprises

(c) contacting at least a portion of said second effluent comprisingxylene with a third catalyst comprising a third molecular sieve having aConstraint Index in the range of about 3 to about 12 under conditionseffective to crack non-aromatic cyclic hydrocarbons in said secondeffluent and form a third effluent comprising xylene; and

(d) recovering xylene from said third effluent.

In a further aspect, the invention resides in a catalyst system adaptedfor transalkylation a C₉+ aromatic hydrocarbon feedstock with a C₆-C₇aromatic hydrocarbon, the catalyst system comprising:

(a) a first catalyst bed comprising (i) a first molecular sieve having aConstraint Index in the range of about 3 to about 12 and (ii) at leastfirst and second different metals or compounds thereof of Groups 6 to 12of the Periodic Table of the Elements having different benzenesaturation activity; and

(b) a second catalyst bed comprising a second molecular sieve having aConstraint Index less than 3;

wherein the weight ratio of said first catalyst to said second catalystis in the range of about 5:95 to about 75:25 and wherein said firstcatalyst bed is located upstream of said second catalyst bed when thecatalyst system is brought into contact with said C₉+ aromatichydrocarbon feedstock and said C₆-C₇ aromatic hydrocarbon in thepresence of hydrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph plotting wt % saturated aromatic rings against reactortemperature for the C₉+ aromatic transalkylation process of Example 1employing a Pt/ZSM-5 catalyst, a sulfided Pt/ZSM-5 catalyst and aPtCu/ZSM-5 catalyst.

DETAILED DESCRIPTION

Described herein are a process and a multi-bed catalyst system forproducing xylene by transalkylation of a heavy aromatic hydrocarbonfeedstock with a C₆ and/or C₇ aromatic hydrocarbon. In particular, thecatalyst system comprises at least two, and optionally three, catalystbeds which are arranged so that a first catalyst bed is located upstreamof the second catalyst bed and, if present, the third catalyst bed islocated downstream of the second catalyst bed, when the catalyst systemis brought into contact with the heavy aromatic hydrocarbon feedstockand the C₆-C₇ aromatic hydrocarbon. The first catalyst bed is effectiveto dealkylate aromatic hydrocarbons in the heavy aromatic feedstockcontaining C₂+ alkyl groups and to saturate the resulting C₂+ olefins,whereas the second catalyst bed is effective to transalkylate the heavyaromatic hydrocarbons with the C₆-C₇ aromatic hydrocarbon to producexylenes. The optional third catalyst bed is effective to cracknon-aromatic cyclic hydrocarbons in effluent from the first and secondcatalyst beds.

Feedstocks

As used herein the term “C_(n)+”, wherein n is a positive integer, meansa compound or group containing at least n carbon atoms. In addition, theterm “C_(n)+ aromatic hydrocarbon feedstock”, wherein n is a positiveinteger, means that a feedstock comprising greater than 50 wt % ofaromatic hydrocarbons having at least n number of carbon atom(s) permolecule.

Thus the heavy aromatic feedstock used in the present process comprisesgreater than 50 wt %, conveniently at least 80 wt %, typically at least90 wt %, of one or more aromatic compounds containing at least 9 carbonatoms. Specific C₉+ aromatic compounds found in a typical feed includemesitylene (1,3,5-trimethylbenzene), durene(1,2,4,5-tetramethylbenzene), hemimellitene (1,2,4-trimethylbenzene),pseudocumene (1,2,4-trimethylbenzene), ethyltoluenes, ethylxylenes,propyl-substituted benzenes, butyl-substituted benzenes, anddimethylethylbenzenes. Suitable sources of the C₉+ aromatics are any C₉+fraction from any refinery process that is rich in aromatics, such ascatalytic reformate, FCC naphtha or TCC naphtha.

The feed to the process also includes benzene and/or toluene, typicallytoluene. The feed may also include unreacted toluene and C₉+ aromaticfeedstock that is recycled after separation of the xylene product fromthe effluent of the transalkylation reaction. Typically, the C₆ and/orC₇ aromatic hydrocarbon constitutes up to 90 wt %, such as from 10 to 70wt % of the entire feed, whereas the C₉+ aromatics component constitutesat least 10 wt %, such as from 30 to 85 wt %, of the entire feed to thetransalkylation reaction.

The feedstock may be characterized by the molar ratio of methyl groupsto single aromatic rings. In some embodiments, the combined feedstock(the combination of the C₉+ and the C₆-C₇ aromatic feedstocks) has amolar ratio of methyl groups to single aromatic rings in the range offrom 0.5 to 4, such as from 1 to 2.5, for example from 1.5 to 2.25.

First Catalyst Bed

The first catalyst bed employed in the present catalyst systemaccommodates a first catalyst comprising a first molecular sieve havinga Constraint Index in the range of about 3 to about 12 and at leastfirst and second different metals or compounds thereof of Groups 6 to 12of the Periodic Table of the Elements.

Constraint Index is a convenient measure of the extent to which analuminosilicate or other molecular sieve provides controlled access tomolecules of varying sizes to its internal structure. For example,molecular sieves which provide a highly restricted access to and egressfrom its internal structure have a high value for the Constraint Index.Molecular sieves of this kind usually have pores of small diameter, e.g.less than 5 Angstroms. On the other hand, molecular sieves which providerelatively free access to their internal pore structure have a low valuefor the Constraint Index, and usually pores of large size. The method bywhich constraint index is determined is described fully in U.S. Pat. No.4,016,218, which is incorporated herein by reference for the details ofthe method.

Suitable molecular sieves for use in the first catalyst comprise atleast one of ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-57 andZSM-58. ZSM-5 is described in detail in U.S. Pat. No. 3,702,886 and Re.29,948. ZSM-11 is described in detail in U.S. Pat. No. 3,709,979. ZSM-22is described in U.S. Pat. Nos. 4,556,477 and 5,336,478. ZSM-23 isdescribed in U.S. Pat. No. 4,076,842. ZSM-35 is described in U.S. Pat.No. 4,016,245. ZSM-48 is more particularly described in U.S. Pat. Nos.4,234,231 and 4,375,573. ZSM-57 is described in U.S. Pat. No. 4,873,067.ZSM-58 is described in U.S. Pat. No. 4,698,217.

In one preferred embodiment, the first molecular sieve comprises ZSM-5and especially ZSM-5 having an average crystal size of less than 0.1micron, such as about 0.05 micron.

Conveniently, the first molecular sieve has an alpha value in the rangeof about 100 to about 1500, such as about 150 to about 1000, for exampleabout 300 to about 600. Alpha value is a measure of the crackingactivity of a catalyst and is described in U.S. Pat. No. 3,354,078 andin the Journal of Catalysis, Vol. 4, p. 527 (1965); Vol. 6, p. 278(1966); and Vol. 61, p. 395 (1980), each incorporated herein byreference as to that description. The experimental conditions of thetest used herein include a constant temperature of 538° C. and avariable flow rate as described in detail in the Journal of Catalysis,Vol. 61, page 395.

Generally, the first molecular sieve is an aluminosilicate having asilica to alumina molar ratio of less than 1000, typically from about 10to about 100.

Typically, the first catalyst comprises at least 1 wt %, preferably atleast 10 wt %, more preferably at least 50 wt %, and most preferably atleast 65 wt %, of the first molecular sieve.

In addition to a molecular sieve having a Constraint Index in the rangeof about 3 to about 12, the first catalyst comprises at least first andsecond different metals or compounds thereof of Groups 6 to 12 of thePeriodic Table of the Elements. As used herein, the numbering scheme forthe Periodic Table Groups is used as in Chemical and Engineering News,63(5), 27 (1985).

The first metal is generally selected from platinum, palladium, iridium,rhenium and mixtures thereof, whereas the second metal is chosen so asto lower the benzene saturation activity of the first metal and isconveniently selected from at least one of copper, silver, gold,ruthenium, iron, tungsten, molybdenum, cobalt, nickel, tin and zinc. Inone embodiment, the first metal comprises platinum and said second metalcomprises copper.

Conveniently, the first metal is present in the first catalyst in anamount between about 0.001 and about 5 wt % of the first catalyst andthe second metal is present in the first catalyst in amount betweenabout 0.001 and about 10 wt % of the first catalyst.

In most cases, the first catalyst also comprises a binder or matrixmaterial that is resistant to the temperatures and other conditionsemployed in the present transalkylation process. Such materials includeactive and inactive materials and synthetic or naturally occurringzeolites, as well as inorganic materials such as clays, silica and/ormetal oxides such as alumina. The inorganic material may be eithernaturally occurring, or in the form of gelatinous precipitates or gelsincluding mixtures of silica and metal oxides. Use of a binder or matrixmaterial which itself is catalytically active, may change the conversionand/or selectivity of the catalyst composition. Inactive materialssuitably serve as diluents to control the amount of conversion so thattransalkylated products can be obtained in an economical and orderlymanner without employing other means for controlling the rate ofreaction. These catalytically active or inactive materials may include,for example, naturally occurring clays, e.g. bentonite and kaolin, toimprove the crush strength of the catalyst composition under commercialoperating conditions.

Naturally occurring clays that can be composited with the firstmolecular sieve as a binder for the catalyst composition include themontmorillonite and kaolin family, which families include thesubbentonites, and the kaolins commonly known as Dixie, McNamee, Georgiaand Florida clays or others in which the main mineral constituent ishalloysite, kaolinite, dickite, nacrite or anauxite. Such clays can beused in the raw state as originally mined or initially subjected tocalcination, acid treatment or chemical modification.

In addition to the foregoing materials, the first molecular sieve can becomposited with a porous matrix binder material, such as an inorganicoxide selected from the group consisting of silica, alumina, zirconia,titania, thoria, beryllia, magnesia, and combinations thereof, such assilica-alumina, silica-magnesia, silica-zirconia, silica-thoria,silica-beryllia, silica-titania, as well as ternary compositions such assilica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesiaand silica-magnesia-zirconia. It may also be advantageous to provide atleast a part of the foregoing porous matrix binder material in colloidalform so as to facilitate extrusion of the catalyst composition.

Typically the first molecular sieve is admixed with the binder or matrixmaterial so that the first catalyst composition contains the binder ormatrix material in an amount ranging from 5 to 95 wt %, and typicallyfrom 10 to 60 wt %.

Second Catalyst Bed

The second catalyst bed accommodates a second catalyst comprising asecond molecular sieve having a Constraint Index less than 3 andoptionally one or more metals or compounds thereof of Groups 6 to 12 ofthe Periodic Table of the Elements.

Suitable molecular sieves for use in the second catalyst compositioncomprise at least one of zeolite beta, zeolite Y, Ultrastable Y (USY),Dealuminized Y (Deal Y), mordenite, NU-87, ZSM-3, ZSM-4 (Mazzite),ZSM-12, ZSM-18, MCM-22, PSH-3, SSZ-25, MCM-36, MCM-49, MCM-56, EMM-10,EMM-10-P and ZSM-20. Zeolite ZSM-4 is described in U.S. Pat. No.3,923,636. Zeolite ZSM-12 is described in U.S. Pat. No. 3,832,449.Zeolite ZSM-20 is described in U.S. Pat. No. 3,972,983. Zeolite Beta isdescribed in U.S. Pat. No. 3,308,069, and Re. No. 28,341. Low sodiumUltrastable Y molecular sieve (USY) is described in U.S. Pat. Nos.3,293,192 and 3,449,070. Dealuminized Y zeolite (Deal Y) may be preparedby the method found in U.S. Pat. No. 3,442,795. Zeolite UHP-Y isdescribed in U.S. Pat. No. 4,401,556. Rare earth exchanged Y (REY) isdescribed in U.S. Pat. No. 3,524,820. Mordenite is a naturally occurringmaterial but is also available in synthetic forms, such as TEA-mordenite(i.e., synthetic mordenite prepared from a reaction mixture comprising atetraethylammonium directing agent). TEA-mordenite is disclosed in U.S.Pat. Nos. 3,766,093 and 3,894,104. MCM-22 is described in U.S. Pat. No.4,954,325. PSH-3 is described in U.S. Pat. No. 4,439,409. SSZ-25 isdescribed in U.S. Pat. No. 4,826,667. MCM-36 is described in U.S. Pat.No. 5,250,277. MCM-49 is described in U.S. Pat. No. 5,236,575. MCM-56 isdescribed in U.S. Pat. No. 5,362,697.

In one preferred embodiment, the second molecular sieve comprises ZSM-12and especially ZSM-12 having an average crystal size of less than 0.1micron, such as about 0.05 micron.

Conveniently, the second molecular sieve has an alpha value of at least20, such as from about 20 to about 500, for example from about 30 toabout 100.

Generally, the second molecular sieve is an aluminosilicate having asilica to alumina molar ratio of less than 500, typically from about 50to about 300.

Typically, the second catalyst comprises at least 1 wt %, preferably atleast 10 wt %, more preferably at least 50 wt %, and most preferably atleast 65 wt %, of the second molecular sieve.

Optionally, the second catalyst comprises at least one and preferably atleast two metals or compounds thereof of Groups 6 to 12 of the PeriodicTable of the Elements. Generally, the second catalyst comprises the samefirst and second metals present in the same amounts as contained by thefirst catalyst.

Generally, the second catalyst also contains a binder or matrixmaterial, which can be any of the materials listed as being suitable forthe first catalyst and can be present in an amount ranging from 5 to 95wt %, and typically from 10 to 60 wt %, of the second catalystcomposition.

Conveniently, the weight ratio of the first catalyst to the secondcatalyst is in the range of 5:95 to 75:25.

Optional Third Catalyst Bed

In addition to the first and second catalysts beds employed in thepresent multi-bed catalysts system, it may be desirable to incorporate athird catalyst bed downstream of the second catalyst bed and effectiveto crack non-aromatic cyclic hydrocarbons in the effluent from the firstand second catalyst beds. The third catalyst bed accommodates a thirdcatalyst comprising a third molecular sieve having a Constraint Indexfrom about 1 to 12. Suitable molecular sieves for use in the thirdcatalyst comprise at least one of ZSM-5, ZSM-11, ZSM-12, zeolite beta,ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-57 and ZSM-58, with ZSM-5 beingpreferred.

Production of the Catalysts

The metal components of the first and second catalysts can beincorporated into the catalyst composition by co-crystallization,exchanged into the composition to the extent a Group 13 element, e.g.,aluminum, is in the molecular sieve structure, impregnated therein, ormixed with the molecular sieve and binder. For example, the metalcomponents can be impregnated in or on the molecular sieve, for examplein the case of platinum, by treating the molecular sieve with a solutioncontaining a platinum metal-containing ion. Suitable platinum compoundsfor impregnating the catalyst with platinum include chloroplatinic acid,platinous chloride and various compounds containing the platinum amminecomplex, such as Pt(NH₃)₄Cl₂H₂O. Alternatively, a compound of thehydrogenation component may be added to the molecular sieve when it isbeing composited with a binder, or after the molecular sieve and binderhave been formed into particles by extrusion or pelletizing. The secondmetal component may be incorporated into the catalyst composition at thesame time and in the same manner as the first metal component.Alternatively, the second metal component may be incorporated into thecatalyst composition after the first metal component has beenincorporated, and this may be achieved in the same or an alternativemanner.

After incorporation of the metal components, the molecular sieve isusually dried by heating at a temperature of 65° C. to 160° C.,typically 110° C. to 143° C., for at least 1 minute and generally notlonger than 24 hours, at pressures ranging from 100 to 200 kPa-a.Thereafter, the molecular sieve may be calcined in a stream of dry gas,such as air or nitrogen, at temperatures of from 260° C. to 650° C. for1 to 20 hours. Calcination is typically conducted at pressures rangingfrom 100 to 300 kPa-a.

Although one advantage of the present multi-bed catalyst system is thatits aromatic hydrogenation activity is low, in some cases it may bedesirable to steam treat and/or sulfide one of more of the catalyst bedsprior to use. Steam treatment may be effected by contacting the catalystcomposition with from 5 to 100% steam at a temperature of at least 260to 650° C. for at least one hour, typically from 1 to 20 hours, at apressure of 100 to 2590 kPa-a. Sulfiding is conveniently accomplished bycontacting the catalyst with a source of sulfur, such as hydrogensulfide, at a temperature ranging from about 320 to 480° C. for a periodof about 1 to about 24 hours.

Transalkylation Apparatus and Process

The first and second catalyst beds and, if present, the third catalystbed may be located in separate reactors but are conveniently located ina single reactor, typically separated from another by spacers or byinert materials, such as, alumina balls or sand. Alternatively, thefirst and second catalyst beds could be located in one reactor and thethird catalyst bed located in a different reactor. As a furtheralternative, the first catalyst bed could be located in one reactor andthe second and third catalyst beds located in a different reactor. Inall situations, the first catalyst is not mixed with the second catalystand the hydrocarbon feedstocks and hydrogen are arranged to contact thefirst catalyst bed prior to contacting the second catalyst bed.Similarly, if the third catalyst bed is present, the hydrocarbonfeedstocks and hydrogen are arranged to contact the second catalyst bedprior to contacting the third catalyst bed.

In operation, the first catalyst bed is maintained under conditionseffective to dealkylate aromatic hydrocarbons containing C₂+ alkylgroups in the heavy aromatic feedstock and to saturate the resulting C₂+olefins. Suitable conditions for operation of the first catalyst bedcomprise a temperature in the range of about 100 to about 800° C.,preferably about 300 to about 500° C., a pressure in the range of about790 to about 7000 kPa-a, preferably about 2170 to 3000 kPa-a, a H₂:HCmolar ratio in the range of about 0.01 to about 20, preferably about 1to about 10, and a WHSV in the range of about 0.01 to about 100 hr⁻¹,preferably about 2 to about 20 hr⁻¹.

The second catalyst bed is maintained under conditions effective totransalkylate C₉+ aromatic hydrocarbons with said at least one C₆-C₇aromatic hydrocarbon. Suitable conditions for operation of the secondcatalyst bed comprise a temperature in the range of about 100 to about800° C., preferably about 300 to about 500° C., a pressure in the rangeof about 790 to about 7000 kPa-a, preferably about 2170 to 3000 kPa-a, aH₂:HC molar ratio in the range of about 0.01 to about 20, preferablyabout 1 to about 10, and a WHSV in the range of about 0.01 to about 100hr⁻¹, preferably about 1 to about 10 hr⁻¹.

Where present, the third catalyst bed is maintained under conditionseffective to crack non-aromatic cyclic hydrocarbons in the effluent fromthe second catalyst bed. Suitable conditions for operation of the thirdcatalyst bed comprise a temperature in the range of about 100 to about800° C., preferably about 300 to about 500° C., a pressure in the rangeof about 790 to about 7000 kPa-a, preferably about 2170 to 3000 kPa-a, aH₂:HC molar ratio in the range of about 0.01 to about 20, preferablyabout 1 to about 10, and a WHSV in the range of about 0.01 to about 100hr⁻¹, preferably about 1 to about 50 hr⁻¹.

Obviously, where the first, second and optional third catalyst beds arelocated in a single reactor, the operating conditions in each bed aresubstantially the same.

The invention will now be more particularly described with reference tothe Examples and accompanying drawing.

EXAMPLES 1 to 3 Dealkylation of Heavy Aromatics Feed Using Bi-metallicMFI Zeolite

A ZSM-5 zeolite having a Si/Al₂ molar ratio of 65 and crystal dimensionsof 0.14 μm was formulated into a 1/16″ cylindrical extrudate (Examples 1and 3) or 1/20″ quadrilobe extrudate (Example 2) using a Versal 300alumina binder such that the mass ratio of zeolite crystal to aluminawas 1:1. A Cu or CuPt solution was added during mulling to create 0.115wt % Pt catalyst (Examples 1 and 3) or 0.115 wt % Pt, 0.0375 wt % Cucatalyst (Example 2). The extrudates were converted into the acidic formby calcining at 538° C. for 2 hours and then steamed at 800° F. (427°C.) for 3 hours in 100% steam to moderate their catalytic activity. Theresultant catalysts were then tested in a fixed-bed microunit. Thereactor pressure was 350 psig (2514 kPa) and the H₂:HC ratio was 2:1.The feed to the reactor contained 85% heavy aromatics and 15%benzene+toluene. A detailed analysis of the feed is shown in Table 1.

TABLE 1 Feed Composition % C5− gas 0.00 Benzene 8.55 Toluene 6.39Ethylbenzene 0.00 Xylenes 0.21 EthylToluene 22.93 Trimethylbenzene 39.09Propylbenzene 2.65 1,n-ethylxylene 10.56 Tetramethylbenzene 2.45 OtherC10 aromatic 5.31 Other C11 aromatic 0.30 Other C12 aromatic 0.00Indanes 0.74 Alkylindanes 0.00 Napthalene 0.01 Alkylnaphthalene 0.00Heavies 0.00 Unidentified 0.80

The catalysts were reduced in hydrogen for 1 hour at 410-420° C. priorto the introduction of feed. The catalysts of Examples 1 and 2 weretested with just the hydrogen reduction prior to feed introduction,whereas the catalyst of Example 3 was subjected to H₂S sulfiding afterhydrogen reduction using 400 ppm H₂S in hydrogen at 420° C. The totalamount of sulfur added to the reactor was 5 moles S per mole Pt prior tofeed introduction, and a further 10 moles S per mole Pt after the feedwas introduced. The activity of the catalysts was determined at 350 psig(2514 kPa) with a H₂:HC molar ratio of 2:1. The total feed flowrate,expressed as grams feed per gram catalyst per hour (WHSV) was 10 hr⁻¹.Product analysis occurred using on-line GC-FID with a 60 m DB-WAXcolumn. The results are summarized in Table 2 and FIG. 1.

TABLE 2 Example 1 2 3 Catalyst Metal 0.115% Pt 0.115% Pt 0.115% Pt0.0375% Cu WHSV 10 10 13.3 Pre-/Co-sulfiding 0/0 0/0 5/10 AverageReactor Temperature, ° F. 786 789 788 Total xylenes 14.6 16.2 13.6Ethylbenzene 0.3 0.7 1.1 PX/Total xylenes, % 24.3 24.2 24.3 PX purity, %23.8 23.2 22.5 Ethyltoluene conversion, % 97.0 95.0 90.5 Ethylxyleneconversion, % 73.6 72.4 58.4 1,3,5-TMB conversion, % −1.8 2.3 −2.01,2,4-TMB conversion, % 21.9 26.6 22.0 1,2,3-TMB conversion, % 29.9 32.223.6 C9 conversion, % 45.9 48.0 43.4 C10 conversion, % 59.3 56.5 48.0C9/10 conversion, % 48.7 49.8 44.3 Toluene + C9/10 conversion, % 28.329.2 25.5 Total TMBs 34.13 32.35 34.43 Total TeMBs 4.43 4.78 4.87 Total218° C.+ BP 0.84 1.41 1.52 % Deethylation 89.7 88.0 80.4 % Depropylation99.4 99.0 98.3 Total saturates 0.11 0.02 0.02 Benzene purity, % 98.8999.81 99.83 Light gas 10.5 8.2 7.3

Comparison of the feed and product compositions in Tables 1 and 2respectively shows that the all catalysts were very effective fordealkylating the feed without catalyzing transalkylation reactions toany significant extent. The Pt-non-sulfided (Example 1) and Pt—Cu system(Example 2) had very similar de-ethylation rates between 88-90%. ThePt-sulfided system (Example 3) was slightly lower at 80%. However, thesaturates produced with the Pt-non-sulfided system (Example 1) were muchhigher at 0.11 wt % than those produced with the Pt—Cu and Pt-sulfidedsystems, which had 0.02 wt % saturates. This point is furtherillustrated in FIG. 1, showing the saturated rings as a function oftemperature for the 3 catalyst systems. As expected, the Pt-non-sulfidedcatalyst (Example 1) saturated increasing more rings as the temperaturedecreased. Both the bimetallic catalyst (Example 2) and the sulfidedPt-only catalyst (Example 3) showed significantly reduced ringsaturation at lower temperatures.

EXAMPLE 4 Dual Bed Heavy Aromatics Transalkylation

The transalkylation of heavy aromatics with benzene and toluene wasdemonstrated over a dual bed catalyst system in a fixed-bed microunit.The following catalyst systems were evaluated:

-   System A    -   Top Bed: 0.115 wt % Pt/50:50 ZSM-5B:Al2O3    -   Mid Bed: 0.1 wt % Pt/65:35 ZSM-12:Al2O3-   System B    -   Top Bed: 0.115 wt % Pt/0.0375 wt % Cu/50:50 ZSM-5B:Al2O    -   Mid Bed: 0.1 wt % Pt/0.0326 wt % Cu/65:35 ZSM-12:Al2O3

The reactor pressure was 350 psig (2514 kPa), the WHSV was 4 and theH₂:HC ratio was 2:1. The feed to the reactor contained 60% heavyaromatics and 40% toluene. A detailed analysis of the feed is shown inTable 3. On start-up, the catalyst beds were reduced in hydrogen at 420°C. The platinum only catalyst system (A) was subjected to H₂S sulfidingafter hydrogen reduction. The total amount of sulfur added to thereactor was 7 moles S per mole Pt prior to feed introduction, and afurther 10 moles S per mole Pt after feed was introduced. Productanalysis was conducted using on-line GC-FID. The results are summarizedin Table 4.

TABLE 3 C5−gas 0.0 Benzene 0.0 Toluene 41.4 Ethylbenzene 0.0 Xylenes 0.1Ethyltoluene 14.0 Trimethylbenzene 29.2 Proplybenzene 1.9 1,n-ethylxylene 6.2 Tetramethylbenzene 2.8 Other C10 aromatics 2.9 Other C11aromatics 0.4 Other C12 aromatics 0.0 Indanes 0.5 Alkylindanes 0.2Napthalene 0.1 Alkynapthalene 0.0 Heavies 0.0 Unidentified 0.2

TABLE 4 Catalyst system A B Pre-/Co-sulfiding 7/10 0/0 Days on stream16.4 6.3 Average reactor temperature ° F. 767 765 Catalyst Performance %Deethylation 70.7 80.6 % Depropylation 95.9 96.7 C2/C2= 1161 2521 C3/C3=875 Benzene purity, % 99.26 99.10 Conversion % Toluene 31.2 32.3 C9 58.160.2 C10 60.5 67.3 Toluene + C9 + C10 47.6 49.4 Ring loss (Naph. = 1ring), % 2.6 3.1 Yields, wt % Light gas (C5−) 7.8 8.7 Non-aromatics 0.30.2 Benzene 6.1 6.5 Toluene 27.5 28.0 Ethylbenzene 1.2 0.8 Xylenes 31.933.5 C9 aromatics 19.6 18.2 MeBenzenes 2.9 1.9 TMB 16.5 16.2 C10aromatics 4.9 4.0 Diethylbenzenes 0.06 0.02 Dimethylethylbenzenes 1.91.3 Tetramethylbenzene 2.6 2.5 C11+ aromatics 1.3 0.9 Naphthalenes 0.50.6

As the data in Table 3 indicates, system B has higherde-ethylation/de-propylation rates and slightly higher conversion withonly slightly higher ring loss (2.6 vs 3.1). If brought to the samede-ethylation and conversion, the ring loss for the two systems would bevery similar, illustrating the suitable use of bi-metallic catalystsystem to replace a platinum only system when sulfiding is notpreferred. The higher ethane to ethylene ratio for the bimetallic systemindicates good metal function for ethylene saturation, which is desired.

While the present invention has been described and illustrated byreference to particular embodiments, those of ordinary skill in the artwill appreciate that the invention lends itself to variations notnecessarily illustrated herein. For this reason, then, reference shouldbe made solely to the appended claims for purposes of determining thetrue scope of the present invention.

1. A process for producing xylene by transalkylation of a C₉+ aromatichydrocarbon feedstock with a C₆ and/or C₇ aromatic hydrocarbon, theprocess comprising: (a) contacting a C₉+ aromatic hydrocarbon feedstock,at least one C₆ and/or C₇ aromatic hydrocarbon and hydrogen with a firstcatalyst under conditions effective to dealkylate aromatic hydrocarbonsin the feedstock containing C₂+ alkyl groups and to saturate C₂+ olefinsformed so as to produce a first effluent, the first catalyst comprising(i) a first molecular sieve having a Constraint Index in the range ofabout 3 to about 12 and (ii) at least first and second different metalsor compounds thereof of Groups 6 to 12 of the Periodic Table of theElements; (b) contacting at least a portion of said first effluent witha second catalyst comprising a second molecular sieve having aConstraint Index less than 3 under conditions effective to transalkylateC₉+ aromatic hydrocarbons with said at least one C₆-C₇ aromatichydrocarbon to form a second effluent comprising xylene; (c) contactingat least a portion of said second effluent comprising xylene with athird catalyst comprising a third molecular sieve having a ConstraintIndex in the range of about 3 to about 12 under conditions effective tocrack non-aromatic cyclic hydrocarbons in said second effluent and forma third effluent comprising xylene.
 2. The process of claim 1, whereinsaid first metal is at least one of platinum, palladium, iridium, andrhenium.
 3. The process of claim 2, wherein said second metal is atleast one of copper, silver, gold, ruthenium, iron, tungsten,molybdenum, cobalt, nickel, tin and zinc.
 4. The process of claim 1,wherein said first metal comprises platinum and said second metalcomprises copper.
 5. The process of claim 2, wherein the first metal ispresent in the first catalyst in an amount between about 0.001 and about5 wt % of the first catalyst.
 6. The process of claim 3, wherein thesecond metal is present in the first catalyst in an amount between about0.001 and about 10 wt % of the first catalyst.
 7. The process of claim1, wherein said first molecular sieve comprises at least one of at leastone of ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-57 and ZSM-58.8. The process of claim 1, wherein said second molecular sieve comprisesat least one of zeolite beta, zeolite Y, Ultrastable Y (USY),Dealuminized Y (Deal Y), mordenite, NU-87, ZSM-3, ZSM-4 (Mazzite),ZSM-12, ZSM-18, MCM-22, MCM-36, MCM-49, MCM-56, EMM-10, EMM-10-P andZSM-20.
 9. The process of claim 1, wherein said first molecular sieve isZSM-5 and said second molecular sieve is ZSM-12.
 10. The process ofclaim 9, wherein said ZSM-5 has a particle size of less than 1 micron,and said ZSM-12 has a particle size of less than 0.5 micron.
 11. Aprocess for producing xylene by transalkylation of a C₉+ aromatichydrocarbon feedstock with a C₆ and/or C₇ aromatic hydrocarbon, theprocess comprising: (a) contacting a C₉+ aromatic hydrocarbon feedstock,at least one C₆ and/or C₇ aromatic hydrocarbon and hydrogen with a firstcatalyst under conditions effective to dealkylate aromatic hydrocarbonsin the feedstock containing C₂+ alkyl groups and to saturate C₂+ olefinsformed so as to produce a first effluent, the first catalyst comprising(i) a first molecular sieve having a Constraint Index in the range ofabout 3 to about 12 and (ii) at least first and second different metalsor compounds thereof of Groups 6 to 12 of the Periodic Table of theElements; and then (b) contacting at least a portion of said firsteffluent with a second catalyst comprising a second molecular sievehaving a Constraint Index less than 3 under conditions effective totransalkylate C₉+ aromatic hydrocarbons with said at least one C₆-C₇aromatic hydrocarbon to form a second effluent comprising xylene;wherein said first metal is at least one of platinum, palladium,iridium, and rhenium; and said second metal is at least one of copper,silver, gold, tungsten, molybdenum, nickel, tin and zinc.
 12. Theprocess of claim 11, wherein the first metal is present in the firstcatalyst in an amount between about 0.001 and about 5 wt % of the firstcatalyst.
 13. The process of claim 11, wherein the second metal ispresent in the first catalyst in an amount between about 0.001 and about10 wt % of the first catalyst.
 14. The process of claim 12, wherein thesecond metal is present in the first catalyst in an amount between about0.001 and about 10 wt % of the first catalyst.
 15. The process of claim11, wherein said first molecular sieve comprises at least one of atleast one of ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-57 andZSM-58.
 16. The process of claim 11, wherein said second molecular sievecomprises at least one of zeolite beta, zeolite Y, Ultrastable Y (USY),Dealuminized Y (Deal Y), mordenite, NU-87, ZSM-3, ZSM-4 (Mazzite),ZSM-12, ZSM-18, MCM-22, MCM-36, MCM-49, MCM-56, EMM-10, EMM-10-P andZSM-20.
 17. The process of claim 12, wherein said second molecular sievecomprises at least one of zeolite beta, zeolite Y, Ultrastable Y (USY),Dealuminized Y (Deal Y), mordenite, NU-87, ZSM-3, ZSM-4 (Mazzite),ZSM-12, ZSM-18, MCM-22, MCM-36, MCM-49, MCM-56, EMM-10, EMM-10-P andZSM-20.
 18. The process of claim 11, wherein said first molecular sieveis ZSM-5 and said second molecular sieve is ZSM-12.
 19. The process ofclaim 18, wherein said ZSM-5 has a particle size of less than 1 micron,and said ZSM-12 has a particle size of less than 0.5 micron.